Acetic acid is a well-known chemical that is available from Lyondell Chemical Company and other producers. Acetic acid is commercially produced by methanol carbonylation in the presence of a rhodium catalyst, methyl iodide, methyl acetate, and water (the “Monsanto process”), see U.S. Pat. No. 3,769,329. Catalyst stabilizers such as lithium iodide or pentavalent Group VA oxides may also be added to the carbonylation reaction. The process results in a high selectivity to acetic acid.
The methanol carbonylation process results in the formation of hydrocarbon impurities such as alkanes and aromatics. The alkanes are typically removed from the process in an alkane distillation column that separates a vapor stream comprising a majority of methyl iodide from a bottoms stream comprising acetic acid, methyl acetate, methyl iodide, and the hydrocarbon impurities. The bottoms stream is typically disposed as waste. Since methyl iodide is an expensive material, previous methods have been disclosed for recovering methyl iodide from the bottoms waste stream. For instance, U.S. Pat. Appl. Pub. No. 2009/0229966 teaches an acetic acid production process that comprises extracting the alkane distillation bottoms stream with water, an acetic acid aqueous solution, or with a methanol aqueous solution, and forming an organic phase comprising the majority of the hydrocarbon impurity and an aqueous phase comprising the majority of water, acetic acid, methyl iodide, and optional methanol. The aqueous phase is recycled to the methanol carbonylation reaction.
Methods for removing alkanes from acetic acid while trying to minimize methyl iodide loss are also known. For instance, U.S. Pat. No. 4,102,922 discloses an alkane removal method in which a slip stream from the heavy phase which comprises methyl iodide, acetic acid, water and alkanes is fed to the alkane distillation column with an overhead temperature of about 75° C. and a bottoms temperature of about 142° C. The bottoms temperature is run significantly higher than the overhead in order to minimize methyl iodide loss to the bottoms stream. The overhead of the alkane distillation, comprising mainly methyl iodide, is recycled to the reaction section. The bottoms stream comprising about 50% acetic acid and about 40% alkanes is removed from the system as waste.
A widely disclosed method teaches removal of iodide impurities from acetic acid by adsorption. A variety of solid adsorbents have been described, and typically contain reactive metals, such as silver, mercury, copper, lead, thallium, palladium, to remove iodide from solution. U.S. Pat. Nos. 4,615,806 and 5,139,981, and 5,227,524 disclose the use of macroreticulated, strong acid cationic exchange resins that contain silver or mercury. The iodide reacts with the resin bound metal and is removed from the acetic acid stream. U.S. Pat. No. 5,220,058 discloses the use of ion exchange resins having metal exchanged thiol functional groups to remove iodide impurities from acetic acid and/or acetic anhydride. The thiol functionality of the ion exchange resin is taught to have been exchanged with silver, palladium, or mercury. European Patent No. 685,445 teaches contacting an iodide containing acetic acid stream with a polyvinylpyridine at elevated temperatures to remove the iodides. U.S. Pat. No. 3,943,229 discloses removing iodide compounds from gaseous streams by contact with certain cross-linked acrylic anion exchange resins. In addition, U.S. Pat. No. 4,650,615 teaches the purification of carboxylic acid anhydrides, such as acetic anhydride, contaminated with halogen and halide values by treating the anhydrides with a phenyl or an alkyl phosphine in the absence of copper, zinc, silver, and cadmium or their compounds and distilling to recover purified anhydrides.
In sum, new methods for the removal of methyl iodide from the alkane distillation bottoms waste stream are needed. We have discovered an effective, convenient method to remove methyl iodide.